The present invention relates to method and apparatus for modifying particles by forming a film of a modifying agent on the surface of particles, and more particularly, to method and apparatus for modifying particles by coating the particles with the modifying agent by letting the modifying agent condense on the surface of the particles under modifying agent""s vaporous atmosphere.
Besides a dry method, a method for modifying the surface of fine particles in a liquid phase has been known as a particle surface modifying method. Also, a kneading method, a stirring method using a medium, a spray dry method, etc. are widely adopted.
The above methods are well-known and described in detail in the references, such as:
1. xe2x80x9cFine Particle Engineering: Fundamentals and Applications of Distributionxe2x80x9d, First Edition, edited by Japan Fine Particle Engineering Technology Association, and published by Asakura Shoten, 1994, pp123-36; and 
2. xe2x80x9cToday""s Chemical Engineering 45 Fine Particle Engineeringxe2x80x9d, edited by Chemical Engineering Association, and published by Chemical Industry Association on Nov. 25, 1993, pp16-23.
Also, Japanese Laid-open Patent Application No. 204545/1995 (Tokukaihei No. 7-204545) discloses a method of forming a film on the surface of a subject. To be more specific, aerosol containing a solvent is sprayed in a chamber to let the aerosol particles collide with the surface of the subject while vaporizing the solvent from the surface of the subject, whereby a film made of solute of the particles is formed on the surface of the subject.
However, according to the above conventional particle modifying method, the particles are electrically charged and become hard to handle thereafter. Moreover, not only the modifying treatment takes too long, but also it demands a complex and expensive apparatus. Further, the above conventional modifying method can hardly modify fine particles, particularly, those in the order of nanometer.
Another method of modifying the particle surface is a method of exposing the particles to a liquid element""s saturated or supersaturated atmosphere at room temperature under atmospheric pressure. For example, Japanese Laid-open Patent Application No. 2865/1977 (Tokukaisho No. 52-2865) discloses, as a technique for capturing aerosol particles, a method for growing aerosol particles up to a predetermined particle size by mixing high-temperature saturated air with low-temperature saturated air, each being a gas containing the aerosol particles, and then letting a water vapor condense on the surface of the aerosol particles. Likewise, an European Patent No. EP 0 794 017 A2 discloses a method for forming a film of a surface modifying agent on the surface of the particles by exposing the particles to a supersaturated atmosphere of the surface modifying agent, and then letting the surface modifying agent condense on the surface of the particles, thereby modifying the surface properties of the particles.
However, the above two methods are silent about an idea of letting a solid surface modifying agent condense on the surface of the particles at room temperature under atmospheric pressure, and the surface modifying agent must be vaporized in each method. Thus, materials which do not vaporize, such as resin (polymer), can not be used as the element for a vapor, and there arises a problem that available materials are limited.
The present invention is devised to solve the above problem, and therefore, has an object to provide method and apparatus for modifying particles which can produce modified particles coated with a thick uniform film of the modifying agent by a very simple manipulation in a short time.
The inventors of the present invention conducted a study to solve the above problem, and has already discovered a particle modifying method as described below, which is totally different from any of the conventional methods.
That is, the inventors of the present invention discovered a particle modifying method for forming a film of the modifying agent on the surface of the particles by the following steps of:
(1) producing a saturated vapor of the modifying agent in a treatment space in a condensing chamber;
(2) exposing the particles to the saturated vapor of the modifying agent by letting the same flow across the treatment space in the condensing chamber; and
(3) turning the saturated vapor of the modifying agent into a supersaturated state by cooling the same in a delivery path connected to the treatment space in the condensing chamber to let the modifying agent condense and form a film on the surface of the particles.
The above method excels the conventional methods because it can modify the surface of the particles by a simple manipulation in a short time without electrically charging the particles. However, the above method needs improvement to form a satisfactory thick uniform film of the modifying agent on the surface of the particles.
Thus, the inventors of the present invention continued an assiduous study to improve the above method and apparatus for modifying the particles, and in due course, discovered following method and apparatus for modifying the particles.
To solve the above problem, a particle modifying method of the present invention has the steps of:
cooling particles to be surface-modified;
forming a vapor of a modifying agent for modifying the particles;
forming a film of the modifying agent on a surface of the particles by (1) producing a supersaturated vapor of the modifying agent around the particles by exposing the particles cooled in the cooling step to the vapor of the modifying agent to be mixed with each other, and (2) letting the modifying agent condense on the surface of the particles.
According to the above method, since the particles cooled in advance are exposed to the vapor of the modifying agent, a large temperature difference is made between the particles and vapor. Thus, a larger amount of the vapor of the modifying agent vapor-deposits on each particle, thereby making it possible to obtain the particles of a relatively large particle size.
Also, since all the particles are cooled to substantially the same temperature before they are introduced into a mixing section, the temperature does not vary much among the particles and is substantially the same. Thus, a substantially equal amount of the vapor of the modifying agent vapor-deposits on each particle, thereby making it possible to obtain the particles of a substantially uniform particle size.
Further, in case that the modifying agent is turned into a supersaturated state by cooling the walls of the mixing section, much of the modifying agent is lost as it deposits onto the wall surface. By contrast, in the present method, the loss of the modifying agent is very small because the supersaturated state is obtained by cooling the particles per se, thereby making it possible to let the modifying agent vapor-deposit on the surface of the particles efficiently. Moreover, in case that particles of a liquid are used, the evaporation of the particles per se when introduced into the saturated vapor of the modifying agent can be suppressed.
Further, since the above method employs a simple technique of cooling the vapor of the modifying agent to obtain the supersaturated atmosphere, it can offer another effect that both the modifying treatment manipulation and apparatus can be simpler.
Furthermore, the above method can form a film of the modifying agent on the surface of the particles without any step such that electrically charges the particles, for example, the stirring of the particles. Thus, the particles after the modifying treatment, namely, the modified particles, are not electrically charged by the modifying treatment, and therefore, easy-to-handle modified particles can be obtained. Also, compared with the stirring of the particles in the conventional particle modifying treatment, the cooling of the particles and the subsequent mixing with the vapor of the modifying agent takes a very short time.
According to the above method, a film of the modifying agent is formed on the surface of the particles by exposing the particles to the supersaturated vapor of the modifying agent and then letting the modifying agent condense on the surface of the particles. Thus, unlike the conventional mixing method or spray dry method, a complex and expensive apparatus can be omitted. Also, the conventional methods have a drawback that the particles are electrically charged, but the above method can modify the particles without electrically charging the particles. Moreover, if a modifying agent having a high degree of supersaturation is selected, a desired amount of the vapor of the modifying agent can be let condense on the surface of the particles without repeating the modifying treatment.
In addition, since the above method uses a simple physical phenomenon, in which a supersaturated atmosphere of the modifying agent is produced and the particles are exposed to the supersaturated atmosphere to let the modifying agent condense on the surface of the particles, the manipulation can be simple and easy and the apparatus for implementing the above method can be simple and inexpensive. Also, surface tension exerts on the film of the modifying agent immediately after it is formed on the surface of the particles. Thus, the particles are coated with a film of the modifying agent having a uniform thickness.
Incidentally, if a modifying agent having a high degree of supersaturation is used, a film of the modifying agent of a desired thickness or modified particles of a desired particle size can be obtained by effecting the particle modifying treatment only once. However, this may not be the case if a modifying agent having a low degree of supersaturation is used.
Thus, in the above particle modifying method, it is preferable to form a thicker film of the modifying agent on the surface of the particles by repeating the film forming step more than once.
According to the above method, since the modifying treatment for forming a film of the modifying agent can be effected repetitively to the same particles, even when a modifying agent having a low degree of supersaturation is used, it becomes possible to grow the particles up to a satisfactory particle size by repeating the modifying treatment as many times as needed. Consequently, the modified particles of a desired particle size coated with a film of the modifying agent of a desired thickness can be obtained, thereby eliminating the above problem.
Also, to solve the above problem, a particle modifying apparatus of the present invention is furnished with:
a mixing section for mixing cooled particles with a vapor of a modifying agent for modifying the particles to let the modifying agent condense on the surface of the particles;
a particle introducing section for introducing the particles into the mixing section;
a cooling device for cooling the particles introduced into the particle introducing section; and
a modifying agent vapor supplying section for supplying the vapor of the modifying agent to the mixing section.
According to the above arrangement, the cooled particles are supplied to a space in the mixing section by the particle introducing section, and the vapor of the modifying agent is supplied to the same by the modifying agent vapor supplying section. Later, the particles and the vapor of the modifying agent are mixed with each other in the mixing section, whereby a supersaturated vapor of the modifying agent is produced around the particles. Consequently, the modified particles having thereon the condensed modifying agent can be obtained.
If the particles are cooled beforehand as described above, the modifying agent can condense around the particles more easily, so that it can form a uniform film on the surface of the particles. Also, in case that particles of a liquid are used, the evaporation of the particles per se when introduced into the saturated vapor of the modifying agent can be suppressed.
Moreover, since the cooled particles are exposed to the vapor of the modifying agent, compared with a case where the particles are introduced without being cooled, there is a larger temperature difference between the particles and vapor of the modifying agent. Thus, a larger amount of the modifying agent vapor-deposits on each particle, and as a consequence, the particles of a larger particle size can be obtained.
Further, since the particles are cooled to substantially the same temperature before they are introduced into the mixing section, the temperature does not vary much among the particles, and is substantially the same. Thus, a substantially equal amount of the modifying agent vapor-deposits on each particle, thereby making it possible to obtain the particles of a substantially uniform particle size.
Furthermore, in case that the modifying agent is turned into a supersaturated state by cooling the mixing section, a loss of the modifying agent is large because much of the vapor of the modifying agent deposits onto the wall surface. By contrast, since the supersaturated state is obtained by cooling the particles per se in the present method, an amount of the vapor of the modifying agent deposited onto the wall surface is very small and so is a loss of the modifying agent, thereby making it possible to let the modifying agent vapor-deposit on the surface of the particles efficiently. Consequently, large particles having a particle size of 5 xcexcm or greater can be obtained. Note that, according to the above arrangement, the modifying treatment can be effected repetitively in succession to the same particles successively introduced into the mixing section from the particle introducing section.
To solve the above problem, another particle modifying apparatus of the present invention is furnished with:
a mixing section for mixing particles with a vapor of a modifying agent for modifying the particles;
a vapor producing chamber for producing the vapor of the modifying agent inside thereof;
a vapor outlet, provided at an end portion of the vapor generating section, for introducing the vapor of the modifying agent in the vapor producing chamber to the mixing section;
a particle inlet, provided to the vapor producing chamber at an end portion on a side of the vapor outlet, for introducing the particles into the vapor producing chamber; and
a cooling device for cooling a mixture of the particles and the vapor of the modifying agent in the mixing section to let the modifying agent condense on the surface of the particles.
According to the above arrangement, since the particle inlet is provided to the vapor producing chamber at an end portion on the side of the vapor outlet, the particles can be transported to the mixing section swiftly without being deteriorated by the heat inside the vapor producing chamber. Also, according to the above arrangement, it has become possible to minimize the vapor-deposition onto the particle introducing section, and hence a drop in concentration of the saturated vapor of the modifying agent filling and flowing in the vapor producing chamber, while at the same time minimizing the temperature interference between the high-temperature interior of the vapor producing chamber and the low-temperature particle introducing section.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.